Local oscillator circuit in superheterodyne receivers



March 31, 1942. K. DOMACK ETAL 8 6] LOCAL OSCILLATOR CIRCUITIN'SUPERHETERODYNE RECEIVER Filed June 5, 1940 Fig.1

. INVENTOR KURT DOMACK +5 'BY/(ARL E GELE ATTORNEY Patented Mar. 31,1942 ammo LOCAL OSCILLATOR CIRCUIT IN SUPER- HETERODYNE RECEIVERS KurtDomack and Karl Eisele, Berlin, Germany,

assignors to Telefunken Gesellschaft fiir Drahtlose Telegraphic m. b.H., Berlin, Germany, a corporation of Germany Application June 5, 1940,Serial No. 338,830 In Germany November 2, 1938 1 Claim.

For the production of oscillations by means of vacuum tubes it is knownto employ the capacitive three-point circuit which is termed alsocapacitive voltage divider circuit or the Colpitts oscillator circuit.This basic circuit can be employed without additional circuit elementsin superheterodyne receivers, wherein for the purpose of obtainingtracking or synchronous operation of the local oscillator circuit andthe input circuit a padding or tracking condenser is placed in series tothe oscillatory coil of the local oscillator. The variable condenser andthe padding condenser are then employed as a capacitive voltage divider.In this case, therefore, a special feed back coil can be dispensed with.The degree of feed back is regulated by the voltage division between thevariable tuning condenser and the padding condenser. Now. since in thecase of short wave reception the padding condenser is to be higher thanin the case of medium waves and long waves, because in the former thepercentage frequency difference of the local oscillator frequencies andinput frequencies is smaller, the potential appearing at the paddingcondenser becomes too low. Therefore, this circuit with capacitivevoltage division cannot be used for short wave reception.

In order to obtain a better understanding of the invention referencewill be had to the accompanying drawing wherein Figure 1 shows anoscillator circuit according to the invention, Figure 2 shows thecircuit of Figure 1 when conditioned, by switch means. to operate atshort waves, and Figure 3 shows the circuit according to Figure 1 whichis effective in the case of reception with intermediatewaves.

In general, according to the invention it is proposed in the medium and/or long wave ranges to employ the variable condenser (C) of the localoscillator and the padding condenser (03) or a part of the latter as acapacitive voltage divider for a feed back three-point circuit, whilefor the short wave range there is provided a feed back coil (Rk) placedin the plate circuit. For the short wave reception there is thenrequired solely a feed back coil while for the medium wave range and.long wave range the feed back coil can be dispensed with.

Now, in this circuit the diiiiculty may be encountered that at thereception of medium waves the oscillator may be excited at a frequencyin the short wave range. Hence, the short wave feed back coil is then tobe rendered inactive for instance by short circuiting it. In accordancewith a. further feature of the invention in order to avoid a specialswitch the capacitively bridged additional long wave coil (L) placed inseries to the grid coils (K and M) for short waves and intermediateWaves is adapted to short circuit the short wave feed back coil (Rk)remaining in the anode line, and this is done also eventually with theadditional padding condenser (C2) for the long wave reception which isplaced in series.

If at long wave reception the danger of a short wave excitation likewiseexists, this condition can be eliminated through a particularconnectionof the parallel capacitance with the additional long wave coilas will be explained below.

Referring now more particularly to Figure 1 there is shown solely thelocal oscillator circuit of asuperheterodyne receiver which can beplaced at the mixer stage in any desired manner. The resistance W4 andthe condenser C1 serve in a manner known as such for the excitation ofthe grid biasing potential which adjusts itself automatically to theamplitude of the local oscillator. In order to obtain a sufficientoscillation potential when employing the said capacitive Voltagedivision, it was found necessary to increase the hitherto customaryvalue of the resistance W4 from about 50,000 ohms to about 150,000 ohms.In order to maintain the time constant of the W4-C1 combination withinadmissible limits whereby any contribution to the appearance ofrelaxation oscillation in the short wave operation is avoided thecapacity of C1 should be reduced from about mmfd. down to 25 mmfd. Theresistance W3 serves in a manner known as such for preventing anincrease of the oscillation potential at the lower end of the short waverange; in this case there appears a reduction of the alternating gridpotential through voltage division between the grid-cathode capacity andthe resistance W3.

At short wave reception only the switch S1 is closed so that the lowerend of the short wave coil K is directly at ground. Between this switchand ground a padding condenser may be inserted which can however bedispensed with at short the circuit elements W3,'W4 and C1 are omittedfor the sake of simplification. It is seen that an inductive returncoupling takes place between the feed back coil Br and the grid circuitcoil K. The circuit elements M, W1, L and C2 of Figure 1 are in parallelto the condenser C3 in the switch position for short wave reception andtherefore, they are practically ineffective and not shown in Figure 2.

At reception on intermediate waves the switch S1 in Figure l is open andthe switch S2 is closed. The part of the circuit according to Figure 1which is effective for the intermediate wave reception is separatelyshown in Figure 3. In contrast to Figure 2 the return coupling takesplace by means of a capacitive three-point circuit.

The short wave feed back coil R1; acts in the case of intermediate wavespractically as a line connection. For this reason, for the intermediatewaves, the long wave additional coil L in Figure 1 and the additionalpadding condenser C2 are practically short-circuited by the switch S2.The parallel capacity which serves to increase the initial capacitancein the case of long wave reception, is for the same reason in parallelto the capacitance C3. By closing the switch S2 there is avoided at thesame time the danger of excitation on a short wave without the use of aspecial switch since the alternating plate current for short waves isby-passed across the switch S2 and the capacity 0 as regards the feedback coil Rk. This would also be the case if the capacity 0 were only inparallel to the coil L. As padding condenser and feed back condenseronly the capacities C3 and c are efiective. A short-circuiting of thesecapacities through the plate current leadin is avoided through theinsertion of the resistance W2 or of a high-frequency choke.

The resistance W1 in the Figures 1 and 3 serves to avoid an increase ofthe amplitude of the local oscillator at'the end of the wave range. Theresistance W1 of about 200 ohms causes a damping action primarily onlyat the upper end of the wave range since then the wattless current inthe oscillatory circuit owing to the higher tuning capacity is largerthan at the lower end of the range and therefore the loss powerincreases. The same resistance W1 fulfills the same task at long wavereception.

If both switches S1 and S2 are open as shown in Figure 1 the circuit isadapted for the long wave reception. In comparison with the intermediatewave reception, in addition to the coil L, the condenser C2 is providedto render the padding capacity lower than in the case of the receptionon intermediate waves. Then the series connection of C2 and C3 iseffective as the padding capacity. However, for the feed back by meansof capacitive voltage division only the condenser C3 is employed becauseotherwise the feed back would be too intense. Thus the circuit isfurther simplified because a special switch contact for reversing theupper end of W2 to have it at the upper end of C2 becomes superfluous.

The danger of excitation on a short wave can,

if necessary, be eliminated if, instead of placing the parallel capacity0 directly in parallel to the coil L, it is grounded at its lower end asshown in Figure 1. In the position of the switches for long wavereception it is not only the feed back coil Rk which produces a feedback action for a short wave but in addition also the feed backpotential appearing at the condenser C3 is to be considered sincewithout employing the said connection of the capacity 0 the lowpotential point of the coil K would for a short wave be connected withthe connection point of C2 and C3 across the capacity b and the capacity0. In View of the particular connection of the capacity 0 the fact isthat for the short wave the low potential point of the coil K is placedat ground across the capacities b and 0 while omitting C3. The capacity0 (about '70 mmfd.) should be high in comparison with the naturalcapacity.

The connection of the capacity 0 has no effect upon the production ofoscillations of the long waves since for the long waves the oscillatorycircuit which is effective consists substantially of the capacity Chaving in parallel thereto the series connection of condensers a, b andc, and also the series connection of the coils K, M and L, and thepadding condensers C2 and C3. It should furthermore be pointed out thatthe capacities a, b and 0 need not necessarily be adjustable.

What we claim is:

In a superheterodyne receiver for the reception of short, medium andlong waves, a local oscillator circuit utilizing a tube having at leasta cathode, grid and anode, a tunable oscillation circuit connectedbetween grid and cathode comprising short, medium and long wave coilsand a pair of padding condensers connected in series and a variabletuning condenser in shunt across said coils and padding condensers, afeed-back coil in coupling relation to only the short wave coilconnected at one end to the anode and at the other end to the commonterminal of the padding condensers, a source of potential havingpositive and negative terminals, the positive terminal being connectedto said common terminal of the padding condensers and the negativeterminal being connected to the cathode of the oscillator, a firstswitch connected from cathode to the junction between the short andmedium wave coils, and a second switch connected from the anode side ofthe feed-back coil to the junction between the medium and long wavecoils, the arrangement being such that with'both switches open thecircuit will oscillate in the long wave band, with only the firstmentioned switch closed the circuit will oscillate in the short waveband, and with only the second mentioned switch closed the circuit willoscillate in the medium wave band.

KURT DOMACK. KARL EISELE.

